Intracerebroventricular delivery of hematopoietic progenitors results in rapid and robust engraftment of microglia-like cells

A versatile mouse model to advance human microglia transplantation research in neurodegenerative diseases

BACKGROUND

Recent studies highlight the critical role of microglia in neurodegenerative disorders, and emphasize the need for humanized models to accurately study microglial responses. Human-mouse microglia xenotransplantation models are a valuable platform for functional studies and for testing therapeutic approaches, yet currently those models are only available for academic research. This hampers their implementation for the development and testing of medication that targets human microglia.

METHODS

We developed the hCSF1Bdes mouse line, which is suitable as a new transplantation model and available to be crossed to any disease model of interest. The hCSF1Bdes model created by CRISPR gene editing is RAG2 deficient and expresses human CSF1. Additionally, we crossed this model with two humanized App KI mice, the AppHu and the AppSAA. Flow cytometry, immunohistochemistry and bulk sequencing was used to study the response of microglia in the context of Alzheimer's disease.

RESULTS

Our results demonstrate the successful transplantation of iPSC-derived human microglia into the brains of hCSF1Bdes mice without triggering a NK-driven immune response. Furthermore, we confirmed the multipronged response of microglia in the context of Alzheimer's disease. The hCSF1Bdes and the crosses with the Alzheimer's disease knock-in model AppSAA and the humanized App knock-in control mice, AppHu are deposited with EMMA and fully accessible to the research community.

CONCLUSION

The hCSF1Bdes mouse is available for both non-profit and for-profit organisations, facilitating the use of the xenotransplantation paradigm for human microglia to study complex human disease.

Lentivirus-modified hematopoietic stem cell gene therapy for advanced symptomatic juvenile metachromatic leukodystrophy: a long-term follow-up pilot study

Metachromatic leukodystrophy (MLD) is an inherited disease caused by a deficiency of the enzyme arylsulfatase A (ARSA). Lentivirus-modified autologous hematopoietic stem cell gene therapy (HSCGT) has recently been approved for clinical use in pre and early symptomatic children with MLD to increase ARSA activity. Unfortunately, this advanced therapy is not available for most patients with MLD who have progressed to more advanced symptomatic stages at diagnosis. Patients with late-onset juvenile MLD typically present with a slower neurological progression of symptoms and represent a significant burden to the economy and healthcare system, whereas those with early onset infantile MLD die within a few years of symptom onset. We conducted a pilot study to determine the safety and benefit of HSCGT in patients with postsymptomatic juvenile MLD and report preliminary results. The safety profile of HSCGT was favorable in this long-term follow-up over 9 years. The most common adverse events (AEs) within 2 months of HSCGT were related to busulfan conditioning, and all AEs resolved. No HSCGT-related AEs and no evidence of distorted hematopoietic differentiation during long-term follow-up for up to 9.6 years. Importantly, to date, patients have maintained remarkably improved ARSA activity with a stable disease state, including increased Functional Independence Measure (FIM) score and decreased magnetic resonance imaging (MRI) lesion score. This long-term follow-up pilot study suggests that HSCGT is safe and provides clinical benefit to patients with postsymptomatic juvenile MLD.

Haploinsufficiency at the CX3CR1 locus of hematopoietic stem cells favors the appearance of microglia-like cells in the central nervous system of transplant recipients

Transplantation of engineered hematopoietic stem/progenitor cells (HSPCs) showed curative potential in patients affected by neurometabolic diseases treated in early stage. Favoring the engraftment and maturation of the engineered HSPCs in the central nervous system (CNS) could allow enhancing further the therapeutic potential of this approach. Here we unveil that HSPCs haplo-insufficient at the Cx3cr1 (Cx3cr1-/+) locus are favored in central nervous system (CNS) engraftment and generation of microglia-like progeny cells (MLCs) as compared to wild type (Cx3cr1+/+) HSPCs upon transplantation in mice. Based on this evidence, we have developed a CRISPR-based targeted gene addition strategy at the human CX3CR1 locus resulting in an enhanced ability of the edited human HSPCs to generate mature MLCs upon transplantation in immunodeficient mice, and in lineage specific, regulated and robust transgene expression. This approach, which benefits from the modulation of pathways involved in microglia maturation and migration in haplo-insufficient cells, may broaden the application of HSPC gene therapy to a larger spectrum of neurometabolic and neurodegenerative diseases.

Therapeutic efficacy of intracerebral hematopoietic stem cell gene therapy in an Alzheimer's disease mouse model

The conditions supporting the generation of microglia-like cells in the central nervous system (CNS) after transplantation of hematopoietic stem/progenitor cells (HSPC) have been studied to advance the treatment of neurodegenerative disorders. Here, we explored the transplantation efficacy of different cell subsets and delivery routes with the goal of favoring the establishment of a stable and exclusive engraftment of HSPCs and their progeny in the CNS of female mice. In this setting, we show that the CNS environment drives the expansion, distribution and myeloid differentiation of the locally transplanted cells towards a microglia-like phenotype. Intra-CNS transplantation of HSPCs engineered to overexpress TREM2 decreased neuroinflammation, Aβ aggregation and improved memory in 5xFAD female mice. Our proof of concept study demonstrates the therapeutic potential of HSPC gene therapy for Alzheimer's disease.

Hematopoietic stem cell gene therapy to halt neurodegeneration

Microglia play fundamental roles in multiple pathological primary and secondary processes affecting the central nervous system that ultimately result in neurodegeneration and for this reason they are considered as a key therapeutic target in several neurodegenerative diseases. Microglia-targeted therapies are directed at either restoring or modulating microglia function, to redirect their functional features toward neuroprotection. Among these strategies, hematopoietic stem cell gene therapy have proven to be endowed with a unique potential for replacing diseased microglia with engineered, transplant progeny cells that can integrate and exert relevant beneficial effects in the central nervous system of patients affected by inherited and acquired neurodegenerative conditions.

Human stem cell transplantation models of Alzheimer's disease

Alzheimer's disease (AD) is the most frequent form of dementia. It is characterized by pronounced neuronal degeneration with formation of neurofibrillary tangles and deposition of amyloid β throughout the central nervous system. Animal models have provided important insights into the pathogenesis of AD and they have shown that different brain cell types including neurons, astrocytes and microglia have important functions in the pathogenesis of AD. However, there are difficulties in translating promising therapeutic observations in mice into clinical application in patients. Alternative models using human cells such as human induced pluripotent stem cells (iPSCs) may provide significant advantages, since they have successfully been used to model disease mechanisms in neurons and in glial cells in neurodegenerative diseases in vitro and in vivo. In this review, we summarize recent studies that describe the transplantation of human iPSC-derived neurons, astrocytes and microglial cells into the forebrain of mice to generate chimeric transplantation models of AD. We also discuss opportunities, challenges and limitations in using differentiated human iPSCs for in vivo disease modeling and their application for biomedical research.

Tagged IDS causes efficient and engraftment-independent prevention of brain pathology during lentiviral gene therapy for Mucopolysaccharidosis type II

Mucopolysaccharidosis type II (OMIM 309900) is a lysosomal storage disorder caused by iduronate 2-sulfatase (IDS) deficiency and accumulation of glycosaminoglycans, leading to progressive neurodegeneration. As intravenously infused enzyme replacement therapy cannot cross the blood-brain barrier (BBB), it fails to treat brain pathology, highlighting the unmet medical need to develop alternative therapies. Here, we test modified versions of hematopoietic stem and progenitor cell (HSPC)-mediated lentiviral gene therapy (LVGT) using IDS tagging in combination with the ubiquitous MND promoter to optimize efficacy in brain and to investigate its mechanism of action. We find that IDS tagging with IGF2 or ApoE2, but not RAP12x2, improves correction of brain heparan sulfate and neuroinflammation at clinically relevant vector copy numbers. HSPC-derived cells engrafted in brain show efficiencies highest in perivascular areas, lower in choroid plexus and meninges, and lowest in parenchyma. Importantly, the efficacy of correction was independent of the number of brain-engrafted cells. These results indicate that tagged versions of IDS can outperform untagged IDS in HSPC-LVGT for the correction of brain pathology in MPS II, and they imply both cell-mediated and tag-mediated correction mechanisms, including passage across the BBB and increased uptake, highlighting their potential for clinical translation.

An innovative hematopoietic stem cell gene therapy approach benefits CLN1 disease in the mouse model

Hematopoietic stem and progenitor cells (HSPCs) can establish a long-lasting microglia-like progeny in the central nervous system of properly myeloablated hosts. We exploited this approach to treat the severe CLN1 neurodegenerative disorder, which is the most aggressive form of neuronal ceroid lipofuscinoses due to palmitoyl-protein thioesterase-1 (PPT1) deficiency. We here provide the first evidence that (i) transplantation of wild-type HSPCs exerts partial but long-lasting mitigation of CLN1 symptoms; (ii) transplantation of HSPCs over-expressing hPPT1 by lentiviral gene transfer enhances the therapeutic benefit of HSPCs transplant, with first demonstration of such a dose-effect benefit for a purely neurodegenerative condition like CLN1 disease; (iii) transplantation of hPPT1 over-expressing HSPCs by a novel intracerebroventricular (ICV) approach is sufficient to transiently ameliorate CLN1-symptoms in the absence of hematopoietic tissue engraftment of the transduced cells; and (iv) combinatorial transplantation of transduced HSPCs intravenously and ICV results in a robust therapeutic benefit, particularly on symptomatic animals. Overall, these findings provide first evidence of efficacy and feasibility of this novel approach to treat CLN1 disease and possibly other neurodegenerative conditions, paving the way for its future clinical application.

Targeting the central nervous system in lysosomal storage diseases: Strategies to deliver therapeutics across the blood-brain barrier

Lysosomal storage diseases (LSDs) are multisystem inherited metabolic disorders caused by dysfunctional lysosomal activity, resulting in the accumulation of undegraded macromolecules in a variety of organs/tissues, including the central nervous system (CNS). Treatments include enzyme replacement therapy, stem/progenitor cell transplantation, and in vivo gene therapy. However, these treatments are not fully effective in treating the CNS as neither enzymes, stem cells, nor viral vectors efficiently cross the blood-brain barrier. Here, we review the latest advancements in improving delivery of different therapeutic agents to the CNS and comment upon outstanding questions in the field of neurological LSDs.

High-throughput analysis of hematopoietic stem cell engraftment after intravenous and intracerebroventricular dosing

Hematopoietic stem/progenitor cell gene therapy (HSPC-GT) has shown clear neurological benefit in rare diseases, which is achieved through the engraftment of genetically modified microglia-like cells (MLCs) in the brain. Still, the engraftment dynamics and the nature of engineered MLCs, as well as their potential use in common neurogenerative diseases, have remained largely unexplored. Here, we comprehensively characterized how different routes of administration affect the biodistribution of genetically engineered MLCs and other HSPC derivatives in mice. We generated a high-resolution single-cell transcriptional map of MLCs and discovered that they could clearly be distinguished from macrophages as well as from resident microglia by the expression of a specific gene signature that is reflective of their HSPC ontogeny and irrespective of their long-term engraftment history. Lastly, using murine models of Parkinson's disease and frontotemporal dementia, we demonstrated that MLCs can deliver therapeutically relevant levels of transgenic protein to the brain, thereby opening avenues for the clinical translation of HSPC-GT to the treatment of major neurological diseases.

Improved engraftment and therapeutic efficacy by human genome-edited hematopoietic stem cells with Busulfan-based myeloablation

Autologous hematopoietic stem cell transplantation using genome-edited cells can become a definitive therapy for hematological and non-hematological disorders with neurological involvement. Proof-of-concept studies using human genome-edited hematopoietic stem cells have been hindered by the low efficiency of engraftment of the edited cells in the bone marrow and their modest efficacy in the CNS. To address these challenges, we tested a myeloablative conditioning regimen based on Busulfan in an immunocompromised model of mucopolysaccharidosis type 1. Compared with sub-lethal irradiation, Busulfan conditioning enhanced the engraftment of edited CD34+ cells in the bone marrow, as well the long-term homing and survival of bone-marrow-derived cells in viscera, and in the CNS, resulting in higher transgene expression and biochemical correction in these organs. Edited cell selection using a clinically compatible marker resulted in a population with low engraftment potential. We conclude that conditioning can impact the engraftment of edited hematopoietic stem cells. Furthermore, Busulfan-conditioned recipients have a higher expression of therapeutic proteins in target organs, particularly in the CNS, constituting a better conditioning approach for non-hematological diseases with neurological involvement.

Therapeutic advantages of combined gene/cell therapy strategies in a murine model of GM2 gangliosidosis

Genetic deficiency of β-N-acetylhexosaminidase (Hex) functionality leads to accumulation of GM2 ganglioside in Tay-Sachs disease and Sandhoff disease (SD), which presently lack approved therapies. Current experimental gene therapy (GT) approaches with adeno-associated viral vectors (AAVs) still pose safety and efficacy issues, supporting the search for alternative therapeutic strategies. Here we leveraged the lentiviral vector (LV)-mediated intracerebral (IC) GT platform to deliver Hex genes to the CNS and combined this strategy with bone marrow transplantation (BMT) to provide a timely, pervasive, and long-lasting source of the Hex enzyme in the CNS and periphery of SD mice. Combined therapy outperformed individual treatments in terms of lifespan extension and normalization of the neuroinflammatory/neurodegenerative phenotypes of SD mice. These benefits correlated with a time-dependent increase in Hex activity and a remarkable reduction in GM2 storage in brain tissues that single treatments failed to achieve. Our results highlight the synergic mode of action of LV-mediated IC GT and BMT, clarify the contribution of treatments to the therapeutic outcome, and inform on the realistic threshold of corrective enzymatic activity. These results have important implications for interpretation of ongoing experimental therapies and for design of more effective treatment strategies for GM2 gangliosidosis.

New Indications for Hematopoietic Stem Cell Gene Therapy in Lysosomal Storage Disorders

Lysosomal storage disorders (LSDs) are a heterogenous group of disorders due to genetically determined deficits of lysosomal enzymes. The specific molecular mechanism and disease phenotype depends on the type of storage material. Several disorders affect the brain resulting in severe clinical manifestations that substantially impact the expectancy and quality of life. Current treatment modalities for LSDs include enzyme replacement therapy (ERT) and hematopoietic cell transplantation (HCT) from allogeneic healthy donors, but are available for a limited number of disorders and lack efficacy on several clinical manifestations. Hematopoietic stem cell gene therapy (HSC GT) based on integrating lentiviral vectors resulted in robust clinical benefit when administered to patients affected by Metachromatic Leukodystrophy, for whom it is now available as a registered medicinal product. More recently, HSC GT has also shown promising results in Hurler syndrome patients. Here, we discuss possible novel HSC GT indications that are currently under development. If these novel drugs will prove effective, they might represent a new standard of care for these disorders, but several challenges will need to be addresses, including defining and possibly expanding the patient population for whom HSC GT could be efficacious.

Continuous human iPSC-macrophage mass production by suspension culture in stirred tank bioreactors

Macrophages derived from human induced pluripotent stem cells (iPSCs) have the potential to enable the development of cell-based therapies for numerous disease conditions. We here provide a detailed protocol for the mass production of iPSC-derived macrophages (iPSC-Mac) in scalable suspension culture on an orbital shaker or in stirred-tank bioreactors (STBRs). This strategy is straightforward, robust and characterized by the differentiation of primed iPSC aggregates into 'myeloid-cell-forming-complex' intermediates by means of a minimal cytokine cocktail. In contrast to the 'batch-like differentiation approaches' established for other iPSC-derived lineages, myeloid-cell-forming-complex-intermediates are stably maintained in suspension culture and continuously generate functional and highly pure iPSC-Mac. Employing a culture volume of 120 ml in the STBR platform, ~1-4 × 107 iPSC-Mac can be harvested at weekly intervals for several months. The STBR technology allows for real-time monitoring of crucial process parameters such as biomass, pH, dissolved oxygen, and nutrition levels; the system also promotes systematic process development, optimization and linear upscaling. The process duration, from the expansion of iPSC until the first iPSC-Mac harvest, is 28 d. Successful application of the protocol requires expertise in pluripotent stem cell culture, differentiation and analytical methods, such as flow cytometry. Fundamental know-how in biotechnology is also advantageous to run the process in the STBR platform. The continuous, scalable production of well-defined iPSC-Mac populations is highly relevant to various fields, ranging from developmental biology, immunology and cell therapies to industrial applications for drug safety and discovery.

Lentiviral haematopoietic stem-cell gene therapy for early-onset metachromatic leukodystrophy: long-term results from a non-randomised, open-label, phase 1/2 trial and expanded access

BACKGROUND

Effective treatment for metachromatic leukodystrophy (MLD) remains a substantial unmet medical need. In this study we investigated the safety and efficacy of atidarsagene autotemcel (arsa-cel) in patients with MLD.

METHODS

This study is an integrated analysis of results from a prospective, non-randomised, phase 1/2 clinical study and expanded-access frameworks. 29 paediatric patients with pre-symptomatic or early-symptomatic early-onset MLD with biochemical and molecular confirmation of diagnosis were treated with arsa-cel, a gene therapy containing an autologous haematopoietic stem and progenitor cell (HSPC) population transduced ex vivo with a lentiviral vector encoding human arylsulfatase A (ARSA) cDNA, and compared with an untreated natural history (NHx) cohort of 31 patients with early-onset MLD, matched by age and disease subtype. Patients were treated and followed up at Ospedale San Raffaele, Milan, Italy. The coprimary efficacy endpoints were an improvement of more than 10% in total gross motor function measure score at 2 years after treatment in treated patients compared with controls, and change from baseline of total peripheral blood mononuclear cell (PBMC) ARSA activity at 2 years after treatment compared with values before treatment. This phase 1/2 study is registered with ClinicalTrials.gov, NCT01560182.

FINDINGS

At the time of analyses, 26 patients treated with arsa-cel were alive with median follow-up of 3·16 years (range 0·64-7·51). Two patients died due to disease progression and one due to a sudden event deemed unlikely to be related to treatment. After busulfan conditioning, all arsa-cel treated patients showed sustained multilineage engraftment of genetically modified HSPCs. ARSA activity in PBMCs was significantly increased above baseline 2 years after treatment by a mean 18·7-fold (95% CI 8·3-42·2; p<0·0001) in patients with the late-infantile variant and 5·7-fold (2·6-12·4; p<0·0001) in patients with the early-juvenile variant. Mean differences in total scores for gross motor function measure between treated patients and age-matched and disease subtype-matched NHx patients 2 years after treatment were significant for both patients with late-infantile MLD (66% [95% CI 48·9-82·3]) and early-juvenile MLD (42% [12·3-71·8]). Most treated patients progressively acquired motor skills within the predicted range of healthy children or had stabilised motor performance (maintaining the ability to walk). Further, most displayed normal cognitive development and prevention or delay of central and peripheral demyelination and brain atrophy throughout follow-up; treatment benefits were particularly apparent in patients treated before symptom onset. The infusion was well tolerated and there was no evidence of abnormal clonal proliferation or replication-competent lentivirus. All patients had at least one grade 3 or higher adverse event; most were related to conditioning or to background disease. The only adverse event related to arsa-cel was the transient development of anti-ARSA antibodies in four patients, which did not affect clinical outcomes.

INTERPRETATION

Treatment with arsa-cel resulted in sustained, clinically relevant benefits in children with early-onset MLD by preserving cognitive function and motor development in most patients, and slowing demyelination and brain atrophy.

FUNDING

Orchard Therapeutics, Fondazione Telethon, and GlaxoSmithKline.

Synthesis and Characterization of a "Clickable" PBR28 TSPO-Selective Ligand Derivative Suitable for the Functionalization of Biodegradable Polymer Nanoparticles

Reactive microgliosis is a pathological hallmark that accompanies neuronal demise in many neurodegenerative diseases, ranging from acute brain/spinal cord injuries to chronic diseases, such as amyotrophic lateral sclerosis (ALS), Alzheimer’s disease (AD) and age-related dementia. One strategy to assess and monitor microgliosis is to use positron emission tomography (PET) by exploiting radioligands selective for the 18 kDa translocator protein (TSPO) which is highly upregulated in the brain in pathological conditions. Several TSPO ligands have been developed and validated, so far. Among these, PBR28 has been widely adopted for PET imaging at both preclinical and clinical levels, thanks to its high brain penetration and high selectivity. For this reason, PBR28 represents a good candidate for functionalization strategies, where this ligand could be exploited to drive selective targeting of TSPO-expressing cells. Since the PBR28 structure lacks functional moieties that could be exploited for derivatization, in this work we explored a synthetic pathway for the synthesis of a PBR28 derivative carrying an alkyne group (PBR-alkyne), enabling the fast conjugation of the ligand through azide-alkyne cycloaddition, also known as click-chemistry. As a proof of concept, we demonstrated in silico that the derivatized PBR28 ligand maintains the capability to fit into the TSPO binding pocked, and we successfully exploited PBR-alkyne to decorate zwitterionic biodegradable polymer nanoparticles (NPs) resulting in efficient internalization in cultured microglia-like cell lines.

Emerging Concepts in Vector Development for Glial Gene Therapy: Implications for Leukodystrophies

Central Nervous System (CNS) homeostasis and function rely on intercellular synchronization of metabolic pathways. Developmental and neurochemical imbalances arising from mutations are frequently associated with devastating and often intractable neurological dysfunction. In the absence of pharmacological treatment options, but with knowledge of the genetic cause underlying the pathophysiology, gene therapy holds promise for disease control. Consideration of leukodystrophies provide a case in point; we review cell type – specific expression pattern of the disease – causing genes and reflect on genetic and cellular treatment approaches including ex vivo hematopoietic stem cell gene therapies and in vivo approaches using adeno-associated virus (AAV) vectors. We link recent advances in vectorology to glial targeting directed towards gene therapies for specific leukodystrophies and related developmental or neurometabolic disorders affecting the CNS white matter and frame strategies for therapy development in future.

Long-Term Follow-Up of Hematopoietic Stem-Cell Gene Therapy for Cerebral Adrenoleukodystrophy

In 2009, cerebral adrenoleukodystrophy (c-ALD) became the first brain disease to be treated with lentiviral (LV)-based hematopoietic stem cell gene therapy with the ABCD1 gene in four boys (P1-P4) who had demyelinating lesions expected to be lethal in the short term and no bone marrow donor. We report the clinical and magnetic resonance imaging (MRI) follow-up over a mean of 8.8 years posttransplant. In parallel, vector genome copies, expression of transgenic ALD protein (ALDP), and viral integration sites were determined in peripheral blood cells. Prior to transplant, the four patients had a normal or near normal neurocognitive status but gadolinium-enhanced demyelination in various brain regions. Gadolinium diffusion disappeared during the first year posttransplant. P3 kept a near normal status until 8.3 years of follow-up, but P1, P2, and P4 showed major cognitive degradation around 9, 28, and 60 months posttransplant. Neurological status and demyelination stabilized until last evaluation in P2, but deteriorated in both P1 at 10 years and P4 at 3 years posttransplant. The proportion of myeloid and lymphoid cells expressing transgenic ALDP decreased by half within 5 years then stabilized around 5% to 10%. Integration site analysis revealed a durable polyclonal distribution of genetically corrected hematopoietic cells. No adverse effects were observed. The long-term arrest of demyelination at MRI and persistence of transduced hematopoietic progenitors support that LV gene therapy may be a safe and durable treatment of c-ALD. However, the neurological degradation observed in three out of four patients mitigates the benefit of this therapy, calling for an earlier intervention, more potent vectors, and additional therapeutic strategies.

Bone marrow-derived myeloid progenitors in the leptomeninges of adult mice

Although the bone marrow contains most hematopoietic activity during adulthood, hematopoietic stem and progenitor cells can be recovered from various extramedullary sites. Cells with hematopoietic progenitor properties have even been reported in the adult brain under steady-state conditions, but their nature and localization remain insufficiently defined. Here, we describe a heterogeneous population of myeloid progenitors in the leptomeninges of adult C57BL/6 mice. This cell pool included common myeloid, granulocyte/macrophage, and megakaryocyte/erythrocyte progenitors. Accordingly, it gave rise to all major myelo-erythroid lineages in clonogenic culture assays. Brain-associated progenitors persisted after tissue perfusion and were partially inaccessible to intravenous antibodies, suggesting their localization behind continuous blood vessel endothelium such as the blood-arachnoid barrier. Flt3^Cre lineage tracing and bone marrow transplantation showed that the precursors were derived from adult hematopoietic stem cells and were most likely continuously replaced via cell trafficking. Importantly, their occurrence was tied to the immunologic state of the central nervous system (CNS) and was diminished in the context of neuroinflammation and ischemic stroke. Our findings confirm the presence of myeloid progenitors at the meningeal border of the brain and lay the foundation to unravel their possible functions in CNS surveillance and local immune cell production.

Gene therapy using haematopoietic stem and progenitor cells

Haematopoietic stem and progenitor cell (HSPC) gene therapy has emerged as an effective treatment modality for monogenic disorders of the blood system such as primary immunodeficiencies and β-thalassaemia. Medicinal products based on autologous HSPCs corrected using lentiviral and gammaretroviral vectors have now been approved for clinical use, and the site-specific genome modification of HSPCs using gene editing techniques such as CRISPR-Cas9 has shown great clinical promise. Preclinical studies have shown engineered HSPCs could also be used to cross-correct non-haematopoietic cells in neurodegenerative metabolic diseases. Here, we review the most recent advances in HSPC gene therapy and discuss emerging strategies for using HSPC gene therapy for a range of diseases.

Update on Clinical Ex Vivo Hematopoietic Stem Cell Gene Therapy for Inherited Monogenic Diseases

Gene transfer into autologous hematopoietic stem progenitor cells (HSPCs) has the potential to cure monogenic inherited disorders caused by an altered development and/or function of the blood system, such as immune deficiencies and red blood cell and platelet disorders. Gene-corrected HSPCs and their progeny can also be exploited as cell vehicles to deliver molecules into the circulation and tissues, including the central nervous system. In this review, we focus on the progress of clinical development of medicinal products based on HSPCs engineered and modified by integrating viral vectors for the treatment of monogenic blood disorders and metabolic diseases. Two products have reached the stage of market approval in the EU, and more are foreseen to be approved in the near future. Despite these achievements, several challenges remain for HSPC gene therapy (HSPC-GT) precluding a wider application of this type of gene therapy to a wider set of diseases while gene-editing approaches are entering the clinical arena.

Human iPSC-derived microglia: A growing toolset to study the brain's innate immune cells

Recent advances in the generation of microglia from human induced pluripotent stem cells (iPSCs) have provided exciting new approaches to examine and decipher the biology of microglia. As these techniques continue to evolve to encompass more complex in situ and in vivo paradigms, so too have they begun to yield novel scientific insight into the genetics and function of human microglia. As such, researchers now have access to a toolset comprised of three unique "flavors" of iPSC-derived microglia: in vitro microglia (iMGs), organoid microglia (oMGs), and xenotransplanted microglia (xMGs). The goal of this review is to discuss the variety of research applications that each of these techniques enables and to highlight recent discoveries that these methods have begun to uncover. By presenting the research paradigms in which each model has been successful, as well as the key benefits and limitations of each approach, it is our hope that this review will help interested researchers to incorporate these techniques into their studies, collectively advancing our understanding of human microglia biology.

Human macrophages and innate lymphoid cells: Tissue-resident innate immunity in humanized mice

Macrophages and innate lymphoid cells (ILCs) are tissue-resident cells that play important roles in organ homeostasis and tissue immunity. Their intricate relationship with the organs they reside in allows them to quickly respond to perturbations of organ homeostasis and environmental challenges, such as infection and tissue injury. Macrophages and ILCs have been extensively studied in mice, yet important species-specific differences exist regarding innate immunity between humans and mice. Complementary to ex-vivo studies with human cells, humanized mice (i.e. mice with a human immune system) offer the opportunity to study human macrophages and ILCs in vivo within their surrounding tissue microenvironments. In this review, we will discuss how humanized mice have helped gain new knowledge about the basic biology of these cells, as well as their function in infectious and malignant conditions. Furthermore, we will highlight active areas of investigation related to human macrophages and ILCs, such as their cellular heterogeneity, ontogeny, tissue residency, and plasticity. In the near future, we expect more fundamental discoveries in these areas through the combined use of improved humanized mouse models together with state-of-the-art technologies, such as single-cell RNA-sequencing and CRISPR/Cas9 genome editing.

Gene-Based Approaches to Inherited Neurometabolic Diseases

In the last decade, the gene therapy (GT) field experienced a renaissance, thanks to crucial understandings and innovations in vector design, stem cell manipulation, conditioning protocols, and cell/vector delivery. These efforts were successfully coupled with unprecedented clinical results of the trials employing the newly developed technology and with the novel establishment of academic-industrial partnerships. A renewed and strengthened interest is rising in the development of gene-based approaches for inherited neurometabolic disorders with severe neurological involvement. Inherited metabolic disorders are monogenetic diseases caused by enzymatic or structural deficiencies affecting the lysosomal or peroxisomal metabolic activity. The metabolic defect can primarily affect the central nervous system, leading to neuronal death, microglial activation, inflammatory demyelination, and axonal degeneration. This review provides an overview of the GT strategies currently under clinical investigation for neurometabolic lysosomal and peroxisomal storage diseases, such as adrenoleukodystrophy and metachromatic leukodystrophy, as well as novel emerging indications such as mucopolysaccharidoses, gangliosidoses, and neuronal ceroid lipofuscinoses, with a comprehensive elucidation of the main features and mechanisms at the basis of a successful GT approach for these devastating diseases.

Development of a Chimeric Model to Study and Manipulate Human Microglia In Vivo

iPSC-derived microglia offer a powerful tool to study microglial homeostasis and disease-associated inflammatory responses. Yet, microglia are highly sensitive to their environment, exhibiting transcriptomic deficiencies when kept in isolation from the brain. Furthermore, species-specific genetic variations demonstrate that rodent microglia fail to fully recapitulate the human condition. To address this, we developed an approach to study human microglia within a surrogate brain environment. Transplantation of iPSC-derived hematopoietic-progenitors into the postnatal brain of humanized, immune-deficient mice results in context-dependent differentiation into microglia and other CNS macrophages, acquisition of an ex vivo human microglial gene signature, and responsiveness to both acute and chronic insults. Most notably, transplanted microglia exhibit robust transcriptional responses to Aβ-plaques that only partially overlap with that of murine microglia, revealing new, human-specific Aβ-responsive genes. We therefore have demonstrated that this chimeric model provides a powerful new system to examine the in vivo function of patient-derived and genetically modified microglia.

Assessing the Impact of Cyclosporin A on Lentiviral Transduction and Preservation of Human Hematopoietic Stem Cells in Clinically Relevant Ex Vivo Gene Therapy Settings

Improving hematopoietic stem and progenitor cell (HSPC) permissiveness to lentiviral vector (LV) transduction without compromising their biological properties remains critical for broad-range implementation of gene therapy as a treatment option for several inherited diseases. This study demonstrates that the use of one-hit ex vivo LV transduction protocols based on either cyclosporin A (CsA) or rapamycin enable as efficient gene transfer as the current two-hit clinical standard into bone marrow-derived CD34⁺ cells while better preserving their engraftment capacity in vivo. CsA was additive with another enhancer of transduction, prostaglandin E2, suggesting that tailored enhancer combinations may be applied to overcome multiple blocks to transduction simultaneously in HSPC. Interestingly, besides enhancing LV transduction, CsA also significantly reduced HSPC proliferation, preserving the quiescent G₀ fraction and the more primitive multipotent progenitors, thereby yielding the highest engraftment levels in vivo. Importantly, no alterations in the vector integration profiles could be detected between CsA and control transduced HSPC. Overall, the present findings contribute to the development of more efficient and sustainable LV gene therapy protocols, underscoring the benefits of scaling down required vector doses, as well as shortening the HSPC ex vivo culture time.

Biodegradable polymeric nanoparticles administered in the cerebrospinal fluid: Brain biodistribution, preferential internalization in microglia and implications for cell-selective drug release

Cell-selective drug release in the central nervous system (CNS) holds great promise for the treatment of many CNS disorders but it is still challenging. We previously demonstrated that polymeric nanoparticles (NPs) injected intra-parenchyma in the CNS can be internalized specifically in microglia/macrophages surrounding the injection site. Here, we explored NPs administration in the cerebrospinal fluid (CSF) to achieve a wider spreading and increased cell targeting throughout the CNS; we generated new NPs variants and studied the effect of modifying size and surface charge on NPs biodistribution and cellular uptake. Intra-cerebroventricular administration resulted in prevalent localization of the NPs in proximity to stem-cell niches, such as around the lateral ventricles, the subventricular zone and the rostral migratory stream. NPs internalization occurred preferentially in brain myeloid cells/microglia. We demonstrated that brain biodistribution and extent of internalization in microglia are influenced by NPs dimensions and can be improved by applying a transient disruption of the blood-brain barrier with mannitol, leading to NPs internalization in up to 25% of brain myeloid/microglia cells. A fraction of the targeted cells was positive for markers of proliferation or stained positive for stemness/progenitor-cell markers such as Nestin, c-kit, or NG2. Interestingly, through these newly formulated NPs we obtained controlled and selective release of drugs otherwise difficult to formulate (such as busulfan and etoposide) to the target cells, preventing unwanted side effects and the toxicity obtained by direct brain delivery of the not encapsulated drugs. Overall, these data provide proof of concept of the applicability of these novel NP-based drug formulations for achieving internalization not only in mature microglia but also possibly in more immature myeloid cells in the brain and pave the way for brain-restricted microglia-targeted drug delivery regimens.

Human Interleukin-34 facilitates microglia-like cell differentiation and persistent HIV-1 infection in humanized mice

Background

Microglia are the principal innate immune defense cells of the centeral nervous system (CNS) and the target of the human immunodeficiency virus type one (HIV-1). A complete understanding of human microglial biology and function requires the cell’s presence in a brain microenvironment. Lack of relevant animal models thus far has also precluded studies of HIV-1 infection. Productive viral infection in brain occurs only in human myeloid linage microglia and perivascular macrophages and requires cells present throughout the brain. Once infected, however, microglia become immune competent serving as sources of cellular neurotoxic factors leading to disrupted brain homeostasis and neurodegeneration.

Methods

Herein, we created a humanized bone-marrow chimera producing human “microglia like” cells in NOD.Cg-Prkdc^scidIl2rg^tm1SugTg(CMV-IL34)1/Jic mice. Newborn mice were engrafted intrahepatically with umbilical cord blood derived CD34+ hematopoietic stem progenitor cells (HSPC). After 3 months of stable engraftment, animals were infected with HIV-1_ADA, a myeloid-specific tropic viral isolate. Virologic, immune and brain immunohistology were performed on blood, peripheral lymphoid tissues, and brain.

Results

Human interleukin-34 under the control of the cytomegalovirus promoter inserted in NSG mouse strain drove brain reconstitution of HSPC derived peripheral macrophages into microglial-like cells. These human cells expressed canonical human microglial cell markers that included CD14, CD68, CD163, CD11b, ITGB2, CX3CR1, CSFR1, TREM2 and P2RY12. Prior restriction to HIV-1 infection in the rodent brain rested on an inability to reconstitute human microglia. Thus, the natural emergence of these cells from ingressed peripheral macrophages to the brain could allow, for the first time, the study of a CNS viral reservoir. To this end we monitored HIV-1 infection in a rodent brain. Viral RNA and HIV-1p24 antigens were readily observed in infected brain tissues. Deep RNA sequencing of these infected mice and differential expression analysis revealed human-specific molecular signatures representative of antiviral and neuroinflammatory responses.

Conclusions

This humanized microglia mouse reflected human HIV-1 infection in its known principal reservoir and showed the development of disease-specific innate immune inflammatory and neurotoxic responses mirroring what can occur in an infected human brain.

Electronic supplementary material

The online version of this article (10.1186/s13024-019-0311-y) contains supplementary material, which is available to authorized users.

Delivering Hematopoietic Stem Cell Gene Therapy Treatments for Neurological Lysosomal Diseases

Neurological lysosomal storage diseases are rare, inherited conditions resulting mainly from lysosomal enzyme deficiencies. Current treatments, such as enzyme replacement therapy and hematopoietic stem cell transplantation, fail to effectively treat neurological disease due to insufficient brain delivery of the missing enzyme. Ex vivo gene therapy approaches to overexpress the missing enzyme in hematopoietic stem cells prior to transplant are an emerging technology that has the potential to offer a viable therapy for patients with these debilitating diseases.

Lipid-induced lysosomal damage after demyelination corrupts microglia protective function in lysosomal storage disorders

Neuropathic lysosomal storage disorders (LSDs) present with activated pro‐inflammatory microglia. However, anti‐inflammatory treatment failed to improve disease pathology. We characterise the mechanisms underlying microglia activation in Niemann–Pick disease type A (NPA). We establish that an NPA patient and the acid sphingomyelinase knockout (ASMko) mouse model show amoeboid microglia in neurodegeneration‐prone areas. In vivo microglia ablation worsens disease progression in ASMko mice. We demonstrate the coexistence of different microglia phenotypes in ASMko brains that produce cytokines or counteract neuronal death by clearing myelin debris. Overloading microglial lysosomes through myelin debris accumulation and sphingomyelin build‐up induces lysosomal damage and cathepsin B extracellular release by lysosomal exocytosis. Inhibition of cathepsin B prevents neuronal death and behavioural anomalies in ASMko mice. Similar microglia phenotypes occur in a Niemann–Pick disease type C mouse model and patient. Our results show a protective function for microglia in LSDs and how this is corrupted by lipid lysosomal overload. Data indicate cathepsin B as a key molecule mediating neurodegeneration, opening research pathways for therapeutic targeting of LSDs and other demyelinating diseases.